Increased borders checks in the wake of the Brussels terror attacks are likely to only last for two weeks, according to a union leader.

Controls have been "stepped up" at checkpoints staffed by British officials but the level cannot be maintained, Lucy Moreton suggested.

It comes as startling images emerged of 26 stowaways being discovered in a lorry stopped by Kent police after it had crossed the Channel.

An independent report, meanwhile, found plans to remove foreign criminals and illegal immigrants were cancelled in 40% of cases.

Ms Moreton, general secretary of ISU, which represents border agency and immigration staff, said checks had been "raised" at border points but would significantly disrupt freight if they continued long-term.

She told BBC Radio 4's Today programme: "There's quite a lot of immediately available money for very high profile types of interventions. We've stepped up controls at the border, and at all borders, but it is maintaining that at a high level for a long period of time and whether there is the political will, or potentially even the necessity to do so."

Asked if there was the political will, she replied: "Experience from past would suggest that it isn't. The increased checks at the border last about two weeks, or that's how long they lasted after the Paris attacks."

Close to 34,000 plane tickets had to be cancelled in 18 months, according to a report from the Independent Chief Inspector of Borders and Immigration.

A lack of security staff to accompany detainees out of the country was a "major constraint", the report said.

Ms Moreton said: "More staff would assist. A large number of flights are missed because we simply can't move detainees from one place to another as we need to. That would inevitably increase the cost of the contract to the Home Office and whether the Home Office would be prepared to pay for that is another matter."

The Home Office has outsourced some of its border and immigration functions to private contractors and the report, covering July-November 2015, examined the escorted and non-escorted removal services provided by private firms.

It found that the Home Office and the companies contracted for the removal services were slow to resolve issues and reach agreement on areas for improvement.

The recorded loss on unused tickets was £1.4 million - equivalent to 4% of the total amount spent on tickets - although records were not always kept, inspectors found.

Home Office figures for October 2014 - March 2015 showed that on average 2.5 tickets were issued for each individual successfully removed.

In the financial year 2014-15, "40% of all planned removals were cancelled".

As enforced removal is resorted to only when someone will not leave voluntarily, efforts to delay or stop deportation, through legal challenges or non-compliance, are commonplace.

The report said: "The Home Office regarded some of the reasons for failed removals to be 'out of (its) control'.

"While this might be true in individual cases at the point of removal, it was unclear what steps were being taken to identify lessons that might be applied by the Home Office and others to reduce 'out of control' failures."

The Home Office accepted the report's recommendations and said it was looking for ways to reduce the cost of pre-departure accommodation at a unit for families subject to enforced removal.

An official response said: "Work is already under way to address the recommendations relating to ticketing and escorting and, as the report acknowledges, these issues are being factored into the re-procurement exercise for both contracts which began in 2015."